Hydrogenated products of block copolymers (hereinafter, referred to as hydrogenated block copolymers) containing a styrenic polymer block and a diene polymer block have a rubber elasticity at ambient temperature, can be plasticized and melted by heating to be easily molded, and moreover excel in a balance of flexibility and mechanical characteristics, so they are used in a wide range of fields such as autoparts, household appliances, wire covering, medical parts, sundry articles and footwear, and in recent years, are highly expected to be alternative materials to vinyl chloride resins because of environmental problems.
However, these styrenic hydrogenated block copolymers are insufficient in the rubber elasticity at high temperatures, for example, at 100° C. or more. Further, these styrenic hydrogenated block copolymers sometimes raise a problem of oil resistance depending on their applications.
Then, several proposals have been made on elastomeric compositions using hydrogenated block copolymers. For example, Patent Document 1, Patent Document 2 and Patent Document 3 propose that by crosslinking an elastomeric composition containing a hydrocarbon oil, an olefinic polymer and the like formulated in a hydrogenated block copolymer by using a crosslinking agent such as an organic peroxide, and a crosslinking aid, the compression set at high temperatures of an obtained elastomeric composition is improved. Further, many methods are known in which a partially hydrogenated block copolymer having residual unsaturated groups of a block copolymer (hereinafter, referred to as partially hydrogenated block copolymer), of which conjugated diene parts of its molecular chain central part are partially hydrogenated, is vulcanized.
However, since though the elastomeric compositions containing a hydrogenated block copolymer have recyclability as a feature of thermoplastic elastomers, mainly soft segments are crosslinked which are composed of conjugated diene monomer blocks of the hydrogenated block copolymer, the rubber elasticity is insufficient, and the compression set at high temperatures comparable to that of vulcanized rubbers have not been achieved. When the crosslinking density is raised for improving the rubber elasticity, obtained compositions have a small elongation, thus not providing preferable ones actually.
Patent Document 4 proposes hydrogenated block copolymers having small blocks of 100 or less in polymerization degree containing unsaturated groups at molecular chain terminals. In an example thereof, a hydrogenated block copolymer composition is exemplified which has small blocks of styrene/isoprene random copolymer at molecular terminals. However, such a hydrogenated block copolymer containing small styrene blocks has a decreased cohesive force of the styrene blocks, often resulting in a large compression set.    Patent Document 1: JP-A-59-131613    Patent Document 2: JP-A-8-225713    Patent Document 3: JP-A-2000-109640    Patent Document 4: JP-B-7-116271